The environment where the signal transmission system of the present invention operates is in a system where the transmission is over a short distance across a barrier. An example of this environment is the use of a signal transmission system with a hearing prosthetic device in which a receiver is implanted within the body to drive an electrode to electrically stimulate the auditory nerve. A transmitting antenna is positioned in close proximity to the receiving antenna but across the cutaneous barrier. Signals representing sound are then transmitted across the cutaneous boundary to the receiver for ultimate delivery to the electrode. The signal transmission system then avoids the use of a percutaneous plug and its attendant potential for infection.
Typically, the transmitter and receiver of the signal transmission system use resonant LC antennas to transmit and receive a small amount of electrical energy over a short distance, e.g. centimeters over the skin boundary.
Signal transmission systems are in use in which the transmitter utilizes a separate oscillator and antenna. In this situation, the resonant frequencies of each are separate and adjustable. The frequency of the oscillator is adjusted to the unloaded resonant frequency of the antenna. The spacing between the transmitter and receiver is then carefully adjusted to be the distance at which "critical coupling" occurs. The "critical coupling" phenomenon is well known in the art. At the point of "critical coupling" a minimum rate of change of the output level of the receiver with respect to variations in the antenna to antenna spacing is observed. Near "critical coupling" and over a small spacing variation, the output of the receiver is relatively invariant.
However, if larger variations in spacing occur between the transmitting antenna and the receiving antenna, the signal transmitted to the receiver will be diminished.
Similarly, if mismatches are evident between the frequency of oscillation and the resonant frequency of its antenna, the signal transmitted to the receiver will be diminished.